Method for cracking hydrocarbon products



y W67 HlRO$HI TOKUHI$A ET AL 3,320,154

METHOD FOR CRACKING HYDROCARBON PRODUCTS Filed May 19, 1964 HYDROCARBMFun NAPHIHA \SWERHEATED STEAM i A I R I 1 5 3 4 United States Patent3,320,154 METHGD FOR CRACKING HYDROCARBON PRODUCTS Hiroshi Tokuhisa,Sendai-shi, Eitaro Ishihara, Kobe, and Toshisada Kinoshita,Fujisawa-shi, Japan, assignors to Michikazu Taheyoshi, Hayama-machi,Miura-gun, Kanagawa-ken, Japan Filed May 19, 1964, Ser. No. 368,654Claims priority, application Japan, May 29, 1963, 38/ 27,936 2 Claims.(Cl. 208--130) This invention relates to apparatus and associatedmethods for obtaining cracked products of a hydrocarbon substance bysubjecting the substance to the combustion product of a hydrocarbon fuelto form a gaseous mixture, and thereafter carrying out quenching of themixture in an adiabatic expansion without heat exchange.

As one of the processes for obtaining from a hydrocarbon product, thepetrochemical raw materials, such as ethylene, propylene, acetylene orthe like, such a method has been heretofore used wherein the hydrocarbonfuel is burned in the presence of oxygen or air in a cracking furnace.The raw material hydrocarbon is injected into a high temperature zone tocarry out the cracking reaction, and the cracked gas and the combustiongas thus gener ated are cooled by heat-exchanging means employing aliquid coolant such as water. For improving the thermal crackingefliciency of the hydrocarbon product as well as preventing undesirablechemical change, such as polymerization of the cracked gas and the like,methods capable of increasing the cooling speed several times fasterthan that of the conventional heat exchanger, have been desired for along time and wherein heat exchange employing a liquid coolant is to beavoided.

On the other hand, the plant for carrying out the thermal cracking ofraw materials for petrochemical industries is extremely expensive andalso from an economical viewpoint, the gas cost becomes higher due tohigh depreciation cost of the plant, and, therefore, large size plantshave been required. If a cooling system can be utilized instead of heatexchange in order to simplify the enormous plant, and the heat contentof the gas mixture consisting of the combustion gas and the cracked gascan be utilized to a maximum degree, the costs of producingpetrochemical raw materials will be remarkably reduced.

It is an object of the present invention to provide a cracking methodwhereby the thermal cracking efficiency for hydrocarbons can be greatlyimproved.

It is a further object of the present invention to provide a crackingmethod wherein the cracked hydrocarbon is prevented from undergoingundesirable chemical reaction, such as polymerization reaction and thelike.

It is still a further object of the present invention to provide acracking method, whereby the thermal cracking of hydrocarbons can becarried out with least energy loss.

In accordance with the invention, after a suitable selection of crackingpressure and temperature has been made, the cooling of the cracked gasgenerated by the cracking of the said hydrocarbon is effected not byheat exchange, such as water cooling and the like, but by carrying outadiabatic expansion of the gas, resulting in an increase in the coolingeffect, while simultaneously the power generated by the said adiabaticexpansion can be utilized if desired.

The present invention provides a method wherein both the cracking ofhydrocarbon and recovering of useful power effected by the adiabaticexpansion unit are combined, and the hydrocarbon fuel undergoescombustion by use of compressed and preheated air, oxygen, or the like,wherein the raw material hydrocarbon product to be cracked undergoesthermal cracking as a result of the high temperature and pressurecombustion gas formed,

3,320,154 Patented May 16, 1967 the high temperature and pressure gasmixture formed during the cracking being sent to an adiabatic expansiondevice to cool the said gas to obtain the cracked products. Cooling iscarried out at an extremely high speed for the cracked hydrocarbon andtherefore any unfavorable reaction, such as polymerization reaction andthe like, is completely prevented, whereby the desired cracked prodnetsare effectively obtained.

Further, according to the present invention, the energy released by theadiabatic expansion of the high temperature gas mixture of thermallycracked hydrocarbon raw material can be effectively recovered, forexample, by a gas turbine, and the overall efliciency can be kept at ahigh degree, for example, by utilizing the turbine for the compressionof air or oxygen for the combustion of the hydrocarbon material.

Furthermore, depending upon the thermal cracking reaction of hydrocarbonin the present invention, a large volume of gas having a hightemperature and high value of average molecular weight can be generated,and therefore, the energy recovered efiiciency can be increased in theadiabatic expansion. Further, in the present invention, if the rawmaterial is hydrocarbon liquid, the power required for pressure increasecarried out prior to heating can be greatly saved, and consequently, theoutput of the adiabatic expansion unit can be increased.

An embodiment of the present invention is diagrammatically illustratedin the attached drawing with regard to a naphtha cracking process,wherein a gas-turbine power generating unit is employed simultaneously.

In an open cycle gas turbine heretofore used in the industry, compressedair is increased in pressure up to about 5 atmospheres by a compressorand charged into a combustion chamber, wherein fuel is injected in anamount corresponding to that of the charged air. The temperature of thefuel is kept at about 1500 C. by subjecting it to a complete combustion.Then, a large amount of air is mixed with the high temperaturecombustion gas to decrease the temperature thereof to about 750 C. Thegas turbine is driven by the gas at the reduced temperature of 750 C.This system is well-known in the art.

It is ideal, with respect to efficiency to directly charge the hightemperature combustion gas into the gas turbine, but it is necessary inorder to avoid deterioration of the turbine to decrease the temperatureof the supplied gas to about 750 C.

On the other hand, when naphtha is cracked by a partial combustionmethod, a decrease in temperature and an increase in gas volume due to amixing dilution and endothermic reaction of the combustion gas and therawmaterial naphtha become an indispensable part in the process, andtherefore, it becomes possible by a suitable selection of the crackingreaction conditions to combine the naphtha cracking method with the gasturbine drive.

The above said problems can be solved by causing the raw materialnaphtha to be thermally cracked by the high temperature and pressurecombustion gas obtained by the combustion of hydrocarbon fuel withcompressed and preheated air, oxygen or the like, the high temperatureand pressure gas mixture formed by the cracking being supplied to a gasturbine. Thus the combustion gas of the fuel hydrocarbon is cooled andpower is generated by the adiabatic expansion at the same time.

The temperatures and pressures of the compressed air (or oxygen),combustion gas, cracked gas and the like in this case should bepreferably similar to those of the gas for driving conventional Wellknown gas turbines. Thus, for example, the combustion gas temperature ispreferably between 1300 to 1500 C., the cracked gas temperature ispreferably between 700 to 800 C., and the pressure is preferably between4 and 10 atmospheres.

This establishes the commercial adaptation of the present invention topractical applications.

The advantage of the present invention as observed in the example issuch that, from the viewpoint of the hydrocarbon cracking process, thecooling effect for the cracked gas can be increased by adiabaticexpansion, whereby the thermal cracking efficiency is increased. Inaddition, on the one hand, the heat content retained in the gas mixturecan be effectively recovered as power and, on the other hand from theviewpoint of generation of power, such heat content of combustion gas assubjected to reduction in temperature from about 1500 to 750 C. withoutany utility can be utilized significantly.

In the drawing, numeral 1 represents an air compressor, wherein the air(or oxygen) at room temperature and atmospheric pressure is subjected toan adiabatic compression to produce compressed and heated air at apressure of about 6 to 7 atmospheres and a temperature of approximately250 C. The compressed and heated air is supplied to a heat exchanger 2,wherein it undergoes heat exchange with the exhaust gas from a turbine5, and the temperature of the air is raised to about 350 C., anddelivered to a combustion chamber 3. .Preheated hydrocarbon fuel iscontinuously injected into the combustion furnace in an amountcorresponding to that of the compressed air for complete combustion ofthe fuel. Combustion is effected at a temperature of about -1500 C. inchamber 3. The high temperature and pressure combustion gas thus formedin combustion chamber 3 is led to a cracking furnace 4. 'Superheatedsteam and naphtha are introduced in furnace 4 in prescribed amounts tocause thermal cracking of the naphtha which has been preheated to about200 C. and which is injected into the furnace. The quantity of naphthaintroduced into furnace 4 is such that the temperature of the gasmixture is regulated to about 700 to 800 C. Then, the gas mixture isdelivered to gas turbine 5, wherein energy of the gas is delivered tothe turbine to drive the same while the gas experiences a reduction inpressure and cooling and is discharged at atmospheric pressure at atemperature of 400 to 450 C. The gas, after being cooled by heatexchange in the heat exchanger 2 is subjected to further utilization ofthe waste heat (not shown) and is then transferred to well known gaspurification and separation means 6.

According to the said method, energy balance for a 23,600 kg./hr.naphtha cracking plant, requires that the output of the turbine be about7600 kw., the required power for compression of the air being about 2300kw. The gas turbine generator terminal efficiency is 15.6% and theoverall thermal efiiciency of plant is about 79%.

Actual operative conditions are shown as follows:

(1) Air:

before compression, 36,400 kg./hr., 1.033 atmospheres, 15 C. aftercompression, 36,400 kg./hr., 6.3 atmospheres,

239 C. after heat exchange, 36,400 kg./hr., 6.3 atmospheres,

350 C. (2) Fuel, 2,450 kg./hr., 200 C. (3) Combustion temperature, 1,500C. (4) Steam quantity, 3,450 kg./hr., 276 C,

4 (5) Heavy naphtha (raw material), 23,600 kg./hr., at

200 C. (6) Cracked gas Before turbine charging, 65,900 kg./hr., 6atmospheres, at 750 C. After turbine exhaust, 65,900 kg./hr. 1.08atmospheres, at 432 C. After heat exchange, 65,900 kg./hr., 1.08atmospheres, at 383 C. (7) Product yield (wt. percent):

Ethylene: 29, propylene: 10, methane: 32, butane: 1, Hydrogen: 15,ethane: 7, propane: 1, butadiene: 2,

butylene: 3

The present invention is of course applicable and practicable not onlyto the use of a gas turbine and naphtha, but also to combinations of anyapparatus for generating power through adiabatic expansion as well asany kind of hydrocarbon product. Apparatus suitable for adiabat icexpansion, may be gas turbines, reciprocating engines, screw-type gasexpansion apparatus, gear-type gas expansion apparatus, Roots-type gasexpansion apparatus and the like. On the other hand, such apparatus,which injects the high temperature and pressure gas by means of nozzles,are widely applicable and practicable without giving introducing seriousdifiiculty.

What is claimed is:

I. A method of driving a gas turbine while cracking naphtha, said methodcomprising compressing an oxygen containing gas in a compressor, mixingthe compressed gas and a hydrocarbon fuel in a combustion chamber andeffecting complete combustion of the fuel in said chamber to produce acombustion gas at a temperature of between 1300 and 1500 C., mixing saidcombustion gas with a mixture of naphtha and superheated steam in acracking furnace to crack the naphtha, the naphtha and superheated steambeing added to the combustion gas in a quantity and a pressure such thatthe temperature of the thus obtained gas mixture is between 700 and 800C., adiabatically expanding the latter gas mixture in a turbine to drivethe same while rapidly cooling the gas mixture to prevent polymerizationthereof, cooling and thereafter separating the cracked constituents ofthe naphtha from the gas mixture after the latter has been dischargedfrom the turbine, and driving said compressor by said turbine.

2. A method as claimed in claim 1 wherein the oxygen containing gas isbrought into heat exchange relationship with gaseous mixture dischargedfrom the turbine prior to said combustion.

References Cited by the Examiner UNITED STATES PATENTS 2,727,932 12/1955Evans et al. 260-683 2,904,502 9/ 1959 Shapliegh 208 2,937,140 5/ 1960Stinson 208340 2,970,107 1/1961 Gilmore 208365 3,124,424 3/1964 Hartleyet a1 2081130 3,241,933 3/1966 Ploum et al. 48196 FOREIGN PATENTS1,102,112 3/1961 Germany.

DELBERT E. GANTZ, Primary Examiner. HERBERT LEVINE, Examiner.

1. A METHOD OF DRIVING A GAS TURBINE WHILE CRACKING NAPHTHA, SAID METHODCOMPRISING COMPRESSING AN OXYGEN CONTAINING GAS IN A COMPRESSOR, MIXINGTHE COMPRESSED GAS AND A HYDROCARBON FUEL IN A COMBUSTION CHAMBER ANDEFFECTING COMPLETE COMBUSTION OF THE FUEL IN SAID CHAMBER TO PRODUCE ACOMBUSTION GAS AT A TEMPERATURE OF BETWEEN 1300 AND 1500*C., MIXING SAIDCOMBUSTION GAS WITH A MIXTURE OA NAPHTHA AND SUPERHEATED STEAM IN ACRACKING FURNACE TO CRACK THE NAPHTHA, THE NAPHTHA AND SUPERHEATED STEAMBEING ADDED TO THE COMBUSTION GAS IN A QUANTITY AND A PRESSURE SUCH THATTHE TEMPERATURE OF THE THUS OBTAINED GAS MIXTURE IS BETWEEN 700 AND800*C., ADIABATICALLY EXPANDING THE LATTER GAS MIXTURE IN A TURBINE TODRIVE THE SAME WHILE RAPIDLY COOLING THE GAS MIXTURE TO PREVENTPOLYMERIZATION THEREOF, COOLING AND THEREAFTER SEPARATING THE CRACKEDCONSTITUENTS OF THE NAPHTHA FROM THE GAS MIXTURE AFTER THE LATTER HASBEEN DISCHARGED FROM THE TURBINE, AND DRIVING SAID COMPRESSOR BY SAIDTURBINE.